What is the only chemical commonly used in slurry form to improve settling of solids?

The correct answer is lime. Lime, or calcium hydroxide, is commonly used in slurry form in various water treatment and industrial processes to improve the settling of solids. When lime is added to water, it forms a slurry that acts as a flocculant. This means it enhances the aggregation of smaller particles into larger flocs, which can then settle out of the water more effectively. The increased flocculation improves the sedimentation process, allowing for clearer water and more efficient removal of suspended solids, making it particularly useful in water treatment operations.

In contrast, sulfuric acid is primarily used for pH control and as a dehydrating agent in various reactions; it does not serve the purpose of aiding solid settling. Caustic soda, while a strong base used to adjust pH and treat certain materials, doesn't effectively promote flocculation in the same manner as lime. Soda ash, or sodium carbonate, can soften water and raise pH but is not specifically utilized for improving the settling of solids in slurry form. Thus, the unique ability of lime to act as an effective flocculant in a slurry form specifically designed for this purpose makes it the correct choice.